The pearlite content is an important metallurgical parameter in as-cast ductile iron. It is generally known that pearlite, which is a eutectoid structure comprised of alternate layers of ferrite and cementite (Fe.sub.3 C), influences the hardness, fatigue properties, wear characteristics and the machinability of a ductile iron casting. Although it is also known that pearlite affects the yield and tensile strength of an iron casting, it is not generally known how the pearlite can be conveniently controlled to approach pearlite contents in excess of 90%.
It is believed the pearlite content is determined by the interaction between the rate at which the casting is cooled after solidification and the chemical composition of the casting. Once the molding procedure is decided for a specific shaped casting, it is difficult to control the cooling rate. The cooling rate is largely determined by the size of the casting in cross-section. Heat treatment may be used to overcome the difficulty, but is usually undesirable because of cost and the extra processing steps required.
Thus, control of the chemical composition through alloying becomes a necessary and desirable method step to control pearlite content. It is desirable because of the advantages of economy and the possibility for more accurate predictability of the pearlite content.
One chemical ingredient that has been found successful in increasing the stabilization of pearlite in gray cast iron is that of sulfur. The pearlite content in a typically gray iron casting is very high because of the presence of sulfur inherent in the gray iron charge materials. In the production of ductile cast iron, however, the presence of sulfur is intentionally avoided because of its effect upon the graphite which inhibits the formation of graphite nodules and thereby lowers the fatigue stress capabilities of the cast iron. Ductile iron is designed to accept stress and thus the sulfur content must be kept at a level which will not interfere with the graphite shape.
Therefore, in as-cast ductile iron castings, the chemistry of the melt is placed under severe limitations which prevent the use of readily known pearlite stabilizers This problem is addressed and solved by the present invention.